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  <div class="question_difficulty">
   难度：Medium
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   <h1 class="question_title">
    96. Unique Binary Search Trees
   </h1>
   <p>
    Given
    <em>
     n
    </em>
    , how many structurally unique
    <strong>
     BST's
    </strong>
    (binary search trees) that store values 1 ...&nbsp;
    <em>
     n
    </em>
    ?
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> 3
<strong>Output:</strong> 5
<strong>Explanation:
</strong>Given <em>n</em> = 3, there are a total of 5 unique BST's:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3
</pre>
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  <div>
   <h1 class="question_title">
    96. 不同的二叉搜索树
   </h1>
   <p>
    给定一个整数
    <em>
     n
    </em>
    ，求以&nbsp;1 ...&nbsp;
    <em>
     n
    </em>
    &nbsp;为节点组成的二叉搜索树有多少种？
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入:</strong> 3
<strong>输出:</strong> 5
<strong>解释:
</strong>给定 <em>n</em> = 3, 一共有 5 种不同结构的二叉搜索树:

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3</pre>
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